Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due, in part, to the large volume occupied by a gas. Because the volume of a gas is so much greater than the volume of a liquid containing the same number of gas molecules, the process of transporting natural gas by means other than pipelines typically includes chilling and/or pressurizing the natural gas in order to liquefy it. However, this contributes to the final cost of the natural gas.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline and middle distillates have been decreasing and supplies are not expected to meet demand in the coming years. Middle distillates typically include heating oil, jet fuel, diesel fuel, and kerosene. Fuels that are liquid under standard atmospheric conditions have the advantage that in addition to their value, they can be transported more economically than natural gas, since they do not require energy, equipment, and expense for liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with oxygen to form syngas, which is a combination of carbon monoxide gas and hydrogen gas. In the second transformation, known as the Fischer-Tropsch process, carbon monoxide is reacted with hydrogen to form organic molecules containing carbon and hydrogen. Those organic molecules containing only carbon and hydrogen are known as hydrocarbons. In addition, other organic molecules containing oxygen in addition to carbon and hydrogen known as oxygenates may be formed during the Fischer-Tropsch process. Hydrocarbons having carbons linked in a straight chain are known as aliphatic hydrocarbons that may include paraffins and/or olefins. Paraffins are particularly desirable as the basis of synthetic diesel fuel.
Typically the Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms, and thus having a range of molecular weights. Thus, the Fischer-Tropsch products produced by conversion of natural gas commonly contain a range of hydrocarbons including gases, liquids and waxes. Depending on the molecular weight product distribution, different Fischer-Tropsch product mixtures are ideally suited to different uses. For example, Fischer-Tropsch product mixtures containing liquids may be processed to yield gasoline, as well as heavier middle distillates. Hydrocarbon waxes may be subjected to an additional processing step for conversion to liquid and/or gaseous hydrocarbons. Thus, in the production of a Fischer-Tropsch product stream for processing to a fuel, it is desirable to maximize the production of high value liquid hydrocarbons, such as hydrocarbons with at least 5 carbon atoms per hydrocarbon molecule (C5+ hydrocarbons).
The Fischer-Tropsch process is commonly facilitated by a catalyst. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. A feed containing carbon monoxide and hydrogen is typically contacted with a catalyst in a reaction zone that may include one or more reactors.
Common reactors include packed bed (also termed fixed bed) reactors, fluidized bed reactors and slurry bed reactors. Originally, the Fischer-Tropsch synthesis was carried out in packed bed reactors. These reactors have several drawbacks, such as temperature control, that can be overcome by gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated multiphase reactors sometimes called “slurry reactors” or “slurry bubble columns,” operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are typically converted to gaseous and liquid products. The gaseous products formed enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered from the suspending liquid by using different techniques like filtration, settling, hydrocyclones, magnetic techniques, etc. Gas-agitated multiphase reactors, or slurry bubble column reactors (SBCRs), inherently have very high heat transfer rates, and therefore, reduced reactor cost. This, and the ability to remove and add catalyst online are some of the principal advantages of such reactors as applied to the exothermic Fischer-Tropsch synthesis. Sie and Krishna (Applied Catalysis A: General 1999, 186, p. 55), incorporated herein by reference in its entirety, give a history of the development of various Fischer Tropsch reactors.
One of the critical design considerations in a SBCR is the recovery of usable liquid products (commonly called waxes in this context) from the reactor slurry. The recovered waxes need to be generally free of catalyst particles before being further processed into commercial products. One problem with the removal of usable liquid products, however, is that catalyst particles are dispersed in the liquid and must be separated from the slurry. It is also often desired to return substantially all of the catalyst particles to the reactor in order to maintain a constant inventory of catalyst in the reactor.
In order to reduce catalyst loses and minimize replacement costs, it is highly desirable to obtain a wax product with a minimum solid content from a Fischer-Tropsch slurry reactor. Some of the early work on catalyst/wax separation techniques involved placing a filter on an external slurry circulation loop is described in an article by M. D. Schlesinger, J. H. Crowell, Max Leva and H. H. Storch titled “Fischer-Tropsch Synthesis in Slurry Phase” from the U.S. Bureau of Mines (Engineering and Process Development, Vol. 43, No. 6, page 1474 to 1479, June 1951). Several methods, such as filtration, magnetic separation and settling, are disclosed in “Status Review of Fischer-Tropsch Slurry Reactor/Catalyst Wax Separation Techniques” prepared for the U.S. Department of Energy, Pittsburgh Energy Technology center by P. Z. Zhou, Burns and Roe Services Corporation, February, 1991. These methods have been developed for separating catalyst particles from Fischer-Tropsch wax but have proved less than ideal or were not deemed commercially viable.
In a commercial SBCR, the severe hydrodynamic conditions inside the reactor, coupled with the desired long lifetime of the catalytic material, can result in catalyst attrition. In the case of some attrition-prone catalyst particles, as the catalyst particles break down over time, sub-particles of various sizes may be created, including very small particles known as “fines,” some of which may even be sub-micron in size. The presence of fines in the reactor tends to greatly reduce the effectiveness of the catalyst-liquid separation system.
In a catalyst-liquid separation system utilizing filtration, the frequency of the backwashing operations affects the solids content of the filtrate thus the filtrate quality. The higher the backwashing frequency the poorer the quality of the filtrate. This is because at the beginning of the filtration cycle, which follows the backwashing and before the cake is established, many particles of size smaller than the substrate nominal opening will traverse it and therefore degrading the filtrate quality.
Thus, there remains a need in the art for methods and apparatus to improve the removal of wax products from a slurry with a high solids content, such as a Fischer-Tropsch slurry. Therefore, the embodiments of the present invention are directed to methods and apparatus, for improved operation of a filtration system for recovering an improved-quality product from a slurry and to extend cycle times of the filtration system, that seek to overcome these and other limitations of the prior art.